The basal ganglia of the brain influences the initiation of movement, and one of the two major output pathways from this structure is through the substantia nigra pars reticulata. We have previously found that many of the cells in the substantia nigra discharge in relation to the initiation of saccadic eye movements. The most likey connection of these cells to oculomotor areas is through an inhibitory connection to the intermediate layers if the superior colliculus. We tested the hypothesis that substantia nigra cells exert a tonic inhibition on superior colliculus cells by injecting an agonist (muscimol) and an antagonist (bicuculline) of the neurotransmitter GABA into the superior colliculus. We then determined the effect of these GABA-related chemicals on the initiation of saccadic eye movements. Muscimol produced changes in saccades to targets in the visual field contralateral to the injection: the latency of the saccade was increased, the saccade became hypometric, and the velocity of the saccade was slow. Injection of bicuculline increased the frequency of saccades to the contralateral field--to the area served by the cells in the superior colliculus near the injection site. In this case, the monkey made saccades to this area even when the target was in the ipsilateral part of the visual field. These experiments are consistent with the hypothesis that the substantia nigra acts to tonically inhibit the superior colliculus since bicuculline probably blocked this tonic inhibition and permitted more saccades whose direction and amplitude corresponded to the movement fields of the superior colliculus cells where the injection was made. Muscimol produced effects consistent with an increase in tonic inhibition since saccades towards the movement field were less effective. This manipulation of a specific transmitter allows the development of hypotheses of the relation of one brain area to another that should facilitate the understanding of the circuitry underlying the initiation of saccadic eye movements.